The present invention relates to a process for increasing the lifetime of an extraction medium containing an organophosphorus acid ester and a hydrocarbon, which extraction medium is used to reprocess spent nuclear fuel and/or breeder materials, and more particularly, to a process for removing from the extraction medium impurities produced by chemical and/or radiolytic decomposition and the undesirable compounds of these substances with radionuclides.
In reprocessing spent nuclear fuels and/or breeder materials, the burnt up fuel elements are treated for the recovery of uranium, in reactors where plutonium is also formed, for the recovery of plutonium, and in reactors where breeder material is also formed for the recovery of breeder material. As a rule, the recovery is carried out by dissolving the fuel elements, usually in nitric acid, to form an aqueous solution containing uranium, plutonium, fission products, and/or breeder materials. The uranium and/or plutonium are then extracted from the aqueous solution by bringing the aqueous solution into contact with an organic solvent. During this extraction, the fission products remain in the aqueous solution. The organic solvent used to effect the extraction is generally present in an extraction medium where it is in admixture with a diluent. After the extraction into the organic extraction medium, the plutonium can be stripped from the organic extract into an aqueous solution and then the uranium can be stripped from the organic extract into an aqueous solution. Upon removal of the plutonium and uranium from the organic extract, the organic extract can be recycled and reused in the process system.
The organic extraction mediums most commonly employed at present comprise a mixture of an organophosphorus acid ester which serves as the active solvent extractant and an aliphatic hydrocarbon (alkane) which serves as a diluent for the solvent. Typical hydrocarbon diluents are homologues in the range of C.sub.10 H.sub.22 to C.sub.13 H.sub.28, with kerosene fractions being particularly suitable. The organophosphorus acid ester solvents which are most commonly employed are trialkyl phosphates which preferably comprise about 3 to 8 carbon atoms among each of its alkyl radicals. The most used solvent extractant is tri-n-butyl phosphate, hereafter referred to as "TBP". A satisfactory composition range for the organic extraction medium is approximately 10% to 40% trialkyl phosphate, by volume, and the remainder diluent, although other ratios can be used.
During contact of the extraction medium with the aqueous, nitric acid solution which contains heavily radioactive fission products and nuclear fuel and/or breeder materials, radiolytic and chemical reactions occur which produce undesirable decomposition products in the extractant medium which unfavorably influence the function of the extraction process. Among other things, these decomposition products form strong complexes with plutonium, which complexes strongly favor the organic phase over an aqueous phase. As a result, these complexes cannot be removed from the organic phase into an aqueous stripping phase so that the plutonium remains in the organic phase and losses of valuable plutonium to the organic phase result. Typical decomposition products which are formed and which give rise to such complexes are dibutyl phosphate (DBP) and monobutyl phosphate (MBP). Further, the pronounced complexing properties of the decomposition products bring about an increased, disturbing extraction of fission products, such as zirconium-95, from the starting aqueous nitric acid phase into the organic phase loaded with the various uranium, plutonium, and like actinides. Moreover, an increase in fission product concentration of the organic phase not only reduces the extraction efficiency for the actinides and the separation efficiencies both of the actinides from the fission products and the actinides from each other and the degree of purity of the individual actinides, but also enhances the radiolytic processes in the organic phase and, in addition, aggravates the phase separation by generating turbidities and colloids in the interface between the organic and aqueous phases. Thus, the increased extraction of fission products into the organic extractant medium causes decontamination factors which are less than adequate and a poor separation of the organic phase from the aqueous phase.
Finally, the decomposition products which accumulate in the extraction medium with increasing use of the extraction medium limit the usefulness of an extraction medium charge or its lifetime, respectively. Then, the spent extraction medium charge must be replaced by a fresh one. To overcome these problems, the extraction medium charges have been purified after use to increase their lifetime and considerably reduce operating costs.
For these reasons, it is necessary to provide a method which removes these decomposition products before the extraction medium is reused. In the past, the process which generally has been used in reprocessing plants for purifying the organic extraction medium includes washing the extraction medium in an aqueous solution of sodium carbonate or sodium hydroxide, or a mixture thereof. Thus, in one known reprocessing plant, for example, the organic extraction mediums are washed with 0.1 M Na.sub.2 CO.sub.3 and then with 0.1 M NaOH, in a 4-stage mixer-separator, at 60.degree. C. In a number of other plants, the processes differ only in the differences in concentration of the solutions, such as, for example, the use of 0.1 to 0.5 M Na.sub.2 CO.sub.3 and 0.1 to 0.25 M NaOH, or in the use of pulsed or packed columns instead of a mixer-separator. The chemical principles of the various prior art processes, however, are substantially the same and have a number of significant drawbacks.
In practice, each individual extraction cycle in a process, such as the Purex process, in which tri-butyl phosphate is used as the solvent, must be followed by washing of the extraction medium (solvent wash). The use of aqueous sodium carbonate and sodium hydroxide wash solutions, however, creates a significant amount of salt-containing, alkali, aqueous waste solutions which must be treated as medium-active waste, must be solidified and finally put into permanent storage. At a large plant, voluminous quantities of solidified waste are formed, which may contain .alpha.-radiators, and lead to considerable problems and high operating costs.
When using an alkali aqueous wash solution, residual quantities of plutonium (IV) are extracted from the organic extraction medium into the alkali aqueous phase and form polymeric hydrolysis products in the aqueous waste wash solution which cannot be redissolved under the subsequent process conditions. Finally, deposits form. Particularly in plants having a high throughput, this produces the danger of criticality.